1. Technical Field
The present invention relates to an exposure determining device and an exposure determining method that are capable of determining whether exposure of an image represented by image data is appropriate.
2. Related Art
When images (photographic images) captured by a digital camera or the like are printed by using a printer or a multifunction machine, printing all the photographed images is very wasteful. That is, since it is likely that the photographed images include images where exposure is insufficient, it is wasteful to print the images where exposure is insufficient.
FIGS. 15A and 15B illustrate examples of an image where exposure is appropriate and an image where exposure is insufficient. FIG. 15A illustrates an image obtained by photographing a firework rising toward a night sky, which illustrates an image where even though a background is dark, since bright portions are scattered over the entire image, exposure is appropriate, and the image is suitable for printing. In contrast, FIG. 15B illustrates an image where even though a portion of the image is only bright, since the other portions are dark, exposure is insufficient, and the image is not suitable for printing.
As such, generally, for some images, such as an image that is obtained by photographing a firework or night view, exposure may be appropriate, while for other images where a portion thereof is bright but the other portions are dark, exposure may be insufficient.
Accordingly, if images where exposure is appropriate can be determined from among a plurality of captured images, only images a user wants to print can be printed.
Meanwhile, as a method of determining whether exposure of an image is excessive or insufficient, for example, methods disclosed in JP-A-9-68764 and JP-A-2002-281337 are known.
Specifically, with respect to an image for which exposure is to be determined, an average value of luminance values of pixels of the image is calculated and it is determined whether exposure of the image is excessive or insufficient, on the basis of the average value.
However, as described above, in the case where with respect to an image for which exposure is to be determined, an average value (hereinafter, also referred to as average luminance value) of luminance values of pixels of the image is calculated and it is determined on the basis of the average value whether exposure of the image is appropriate or insufficient, an erroneous determination may be performed for each image.
FIGS. 16A and 16B are diagrams illustrating a histogram of luminance values and an average luminance value for each of images shown in FIGS. 15A and 15B. Specifically, FIG. 16A shows a histogram of luminance values for an image shown in FIG. 15A where exposure is appropriate, and FIG. 16B shows a histogram of luminance values for an image shown in FIG. 15B where exposure is insufficient. In each of FIGS. 16A and 16B, reference character Av indicates an average value of luminance values (average luminance value) of pixels of the image.
As shown in FIGS. 16A and 16B, if an average luminance value Av of pixels of an image shown in FIG. 15A where exposure is appropriate is compared with an average luminance value Av of pixels of an image shown in FIG. 15B where exposure is insufficient, the respective average luminance values Av are substantially the same. Accordingly, it can be understood that if the average luminance values Av are only used, it is difficult to determine that the image shown in FIG. 15A is an image where exposure is appropriate and the image shown in FIG. 15B is an image where exposure is insufficient.